1. Field
This disclosure relates to a negative active material for a rechargeable lithium battery and a rechargeable lithium battery including the same.
2. Description of the Related Art
Lithium rechargeable batteries have recently drawn attention as a power source for small portable electronic devices. They use an organic electrolyte solution and thereby have twice the discharge voltage of a conventional battery using an alkali aqueous solution, and accordingly have high energy density.
For positive active materials of a rechargeable lithium battery, lithium-transition element composite oxides being capable of intercalating lithium, such as LiCoO2, LiMn2O4, LiNi1-xCoxO2 (0<x<1), and so on, have been researched.
As for negative active materials of a rechargeable lithium battery, various carbon-based materials such as artificial graphite, natural graphite, and hard carbon, which can all intercalate and deintercalate lithium ions, have been used. Since graphite among the carbon-based materials has a low discharge potential relative to lithium of −0.2V, a battery using the graphite as a negative active material has a high discharge potential of 3.6V and excellent energy density. Furthermore, graphite guarantees a long cycle life for a battery due to its outstanding reversibility. However, a graphite active material has low density (theoretical density of 2.2 g/cc) and consequently low capacity in terms of energy density per unit volume when the graphite is used as a negative active material. Further, it has swelling or capacity reduction problems when a battery is misused or overcharged and the like, because graphite is likely to react with an organic electrolyte at a high discharge voltage.
In addition, there has been an attempt to use lithium titanate as a negative electrode material. Since lithium titanate has a voltage of 1.5V based on a lithium metal, a long cycle-life, and a higher operation voltage than reduction potential of lithium, it has a merit of preventing lithium extraction on the surface of a negative electrode when overcharged. Accordingly, it is paid attention to as an active material for a large capacity battery.
In particular, when Li4Ti5O12 having a spinel structure repetitively intercalates/deintercalates lithium as a useful negative active material, it is known to have a small crystal structure change and little degradation caused by charge and discharge cycles. However, since Li4Ti5O12 has low electrical conductivity (˜10−9 S/cm), it has a problem of high reaction resistance during the intercalation/deintercalation of lithium and remarkable characteristic deterioration of sharp charge/discharge. Thus, it may not be applied to a battery requiring high power.
Accordingly, in order to improve performance of the lithium titanate as a negative active material for a high output rechargeable battery, much research on reducing its primary particle diameter to increase a specific surface area and make intercalation/deintercalation of lithium ions easy has been made. However, when the specific surface area is excessively increased, side reactions of gas generation may occur.